MINERAL INCLUSIONS IN IGNEOUS ZIRCON IN TEXTURAL AND GEOCHEMICAL CONTEXT

Igneous zircon is not only a robust geochronometer but also reflects the trace element content of its source magma and incorporates mineral inclusions inherited from the magma during growth. All of these clues help constrain the origins of detrital and other out-of-context zircon, with applications ranging from tectonic reconstructions to studies of Earth’s earliest crust. Although mineral inclusions have been less studied as tracers, various oxide and phosphate inclusions may be useful for determining the composition of detrital zircon source granitoids. For example, apatite-poor (i.e., <20% of assemblage) zircon suites mainly derive from highly silicic melts (Bell et al., 2018, Chem. Geo.). This project attempted to better constrain the origins of apatite-rich zircons, which appear to derive from a variety of sources (Bell et al., 2018, Chem. Geo.; Rasmussen et al., 2011, Geology) without a clear correlation between apatite content and source rock character. Relative inclusion abundances in igneous accessory minerals should be affected by 1) crystallization sequence, 2) proximity to other crystallizing phases, and 3) overall magmatic composition. We have selected several plutonic units in the Peninsular Ranges Batholith (PRB) of southern California with zircons rich in apatite in which whole rock chemistry, zircon trace element chemistry, and several robust accessory minerals in a clear crystallization sequence elucidate the relative importance of these three factors. In these PRB granitoids, earlier crystallizing phases have proportionally larger contents of apatite inclusions and lower contents of late-crystallizing phases (i.e., quartz, K-feldspar, muscovite), consistent with textural evidence of apatite as an early-crystallizing phase. Ilmenite inclusion assemblages are shifted toward higher contents of mafic minerals relative to zircon and hornblende. Contents of apatite and late-crystallizing phases in inclusion assemblages appear to be more correlated with the first two factors than more directly with melt composition. Within the zoned La Posta pluton in particular, compositional evolution of several units as shown by zircon trace element chemistry may correlate more directly with the resulting apatite and late phase inclusion contents than whole rock composition.